An exhaust system of a diesel engine that comprises a NOx adsorber catalyst is capable of adsorbing substantial amounts of oxides of nitrogen (NOx) in engine exhaust gases passing through the exhaust system from the engine. The NOx adsorber catalyst thereby reduces the amount of NOx entering the atmosphere, preventing the trapped NOx from contributing to what might otherwise become smog.
Use of a NOx adsorber catalyst has the potential for enabling future diesel engines to meet the requirements of increasingly stringent tailpipe emission regulations promulgated by the United States Environmental Protection Agency.
When a NOx adsorber catalyst is present in the exhaust system of a motor vehicle powered by a diesel engine, it is desirable to regenerate the NOx adsorber catalyst from time to time to remove trapped NOx so that the catalyst can continue to be effective. Regeneration is typically performed only when prevailing conditions are suitable. When a certain level of adsorbed NOx is reached, regeneration is forced. The products of regeneration are non-pollutants that are naturally present in the atmosphere.
A NOx adsorber catalyst uses high levels of CO to accomplish its regeneration. Those high levels can be realized by post-injection of fuel, meaning one or more injections that occur after a main fueling injection into a cylinder during an engine cycle.
Thus, a diesel engine that has NOx adsorber catalyst in its exhaust system runs in either of what may be considered two types of conditions: 1) lean operating condition where the engine is fueled in the usual manner; and 2) rich operating condition where the engine is fueled with post-injection in order to regenerate the NOx adsorber catalyst. It is during regeneration that rich modulation occurs.
During regeneration it is desirable that the engine torque be kept substantially the same as would be in the absence of regeneration so that an operator of the vehicle will not notice the regeneration. Also, the rich modulation that occurs during regeneration should not create undesired torque fluctuations.
It should be apparent however that the addition of a post-injection of fuel in order to richen the air-fuel mixture being combusted during the engine cycle will cause engine torque to increase.
In order to make regeneration transparent to the vehicle operator insofar as the effect of post-injection on engine torque is concerned, the amount of fuel injected during an engine cycle into a cylinder prior to the added post-injection needs to be reduced.